A typical insulated electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric materials including an inner semiconducting shield layer (conductor or strand shield), an insulating layer, an outer semiconducting shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket. Additional layers within this construction such as moisture impervious materials, are often incorporated. The invention pertains to the outer semiconducting insulation shield layer, i.e., the insulation shield and cables made with the outer semiconducting insulation shield in accordance with the invention.
In general, semiconducting dielectric insulation shields can be classified into two distinct types, the first type being a type wherein the dielectric shield is securely bonded to the polymeric insulation so that stripping the dielectric shield is only possible by using a cutting tool that removes the dielectric shield alone with some of the cable insulation. This type of dielectric shield is preferred by companies that believe that this adhesion minimizes the risk of electric breakdown at the interface of the shield and insulation. The second type of dielectric shield is the “strippable” dielectric shield wherein the dielectric shield has a defined, limited, adhesion to the insulation so that the strippable shield can be peeled cleanly away from the insulation without removing any insulation. Current strippable shield compositions for use over insulation materials selected from polyethylene, cross-linked polyethylenes, or one of the ethylene copolymer rubbers such as ethylene-propylene rubber (EPR) or ethylene-propylene diene terpolymer (EPDM) are usually based on an ethylene-vinyl acetate (EVA) copolymer base resin rendered conductive with an appropriate type and amount of carbon black.
Strippable shield formulations of EVA and nitrile rubbers have been described by Ongchin, U.S. Pat. Nos. 4,286,023 and 4,246,142; Burns et al. EP Application No. 0,420,271B, Kakizaki et al U.S. Pat. No. 4,412,938 and Janssun, U.S. Pat. No. 4,226,823, each reference being herein incorporated by reference into this application. A problem with these strippable shield formulations of EVA and nitrile rubber is that the EVA's needed for this formulation have a relatively high vinyl acetate content to achieve the desired adhesion level with the result that the formulations are more rubbery than is desired for high speed extrusion of a commercial electric cable.
Alternative adhesion-adjusting additives have also been proposed for use with EVA, for example waxy aliphatic hydrocarbons (Watanabe et al. U.S. Pat. No. 4,933,107, herein incorporated by reference); low-molecular weight polyethylene (Burns Jr., U.S. Pat. No. 4,150,193 herein incorporated by reference); silicone oils, rubbers and block copolymers that are liquid at room temperature (Taniguchi et al. U.S. Pat. No. 4,493,787 herein incorporated by reference); chlorosulfonated polyethylene, ethylene-propylene rubbers, polychloroprene, styrene-butadiene rubber, and natural rubber. However, the only adhesion-adjusting additives that appear to have found commercial acceptance have been paraffin waxes.
U.S. Pat. No. 6,284,374 to Yamazaki, et al discloses a multi-component polymer composition for use in strippable semiconductive shields suitable for a polyolefin-insulated wire and cable crosslinked by silane grafting/water crosslinking. The main polymer component of the composition is mainly composed of an ethylene/vinyl acetate copolymer having a weight average molecular weight not less than 300,000.
Commonly assigned U.S. Pat. Nos. 6,274,066 and 6,013,202 disclose a strippable semiconductive shield made from a base polymer and an adhesion modifying additive.
US Published Patent Application 2004/0217329A1 to Easter discloses a two component base polymer together with adhesion adjusting additives
WO 2004/088674 A1 to Person discloses a strippable semiconductive shield made from a base polymer which is a soft polymer and a hard polymer.
The use of amide wax additives in a conductor shield has been proposed in commonly assigned U.S. Pat. No. 6,491,849 to Easter to improve aging characteristics of the electric cable.
In the manufacture of commercial quantities of electric cable, minor cost improvements to polymeric compositions where the resulting composition and/or cable employing the composition have acceptable physical or electric properties are considered significant advances in the art. This is because the competitive environment places great demands on product pricing as well as performance and longevity. An improvement which not only reduces cost but improves properties is considered extremely significant, as it positively impacts both cost and quality.
It would be desirable to develop lower cost, easier to compound, strippable semiconductive insulation shield compositions. Other proposals require complicated compounding methods or additives that are, on average, twice as expensive as the base polymers when used to achieve lower adhesion and/or strippability.
A novel two component adhesion modifying additive system for strippable insulation shields is proposed which provides remarkable adhesion results while also improving cost over earlier systems.